There has been defined in the standards organization 3GPP a radio communication system employing an LTE (Long Term Evolution) scheme (hereinafter referred to as “LTE system”) as successor system to the UMTS (Universal Mobile Telecommunications System). Now in 3GPP, a radio communication system employing an LTE-Advanced scheme (hereinafter referred to as “LTE-A system”) has been under study as a successor system to the LTE system.
In the LTE-A system, there has been studied HetNet (Heterogeneous Network) in which a micro cell (for example, pico cell or femto cell) having a local coverage of about several-ten-meter radius is formed in a macro cell having a wide coverage of several-kilometer radius (for example, see Non Patent Literature 1).
In such HetNet, for the purpose of improving throughput of the whole system, it has been studied to perform CRE (Cell Range Expansion). In CRE, the range of the micro cell is expanded by adding an offset to reception power from a radio base station that forms the micro cell (hereinafter referred to as “micro base station”). Therefore, a user terminal positioned inside the expanded micro cell can be handed over from a radio base station that forms the macro cell (hereinafter referred to as “macro base station”) to the micro base station.
Also, in such HetNet, the user terminal handed over to the micro base station by CRE suffers from large interference from the macro base station. Therefore, interference coordination has been under study to stop data transmission by the macro base station in some subframes thereby to reduce interference that the user terminal suffers from by the macro base station.
FIG. 1 is a diagram illustrating an example of interference coordination. As illustrated in FIG. 1, in subframes in which the macro base station performs data transmission (first and third subframes from the left), reception power of the user terminal from the micro base station is lowered because it suffers from interference from the macro base station. On the other hand, in subframes where the macro base station stops data transmission (second and fourth subframes from the left), reception power of the user terminal from the micro base station is increased because it does not suffer from interference from the macro base station. Here, in subframes where the data transmission is stopped (hereinafter referred to as “transmission stopped subframes), data transmission may not be stopped completely or a small amount of data may be transmitted as far as interference to the user terminal falls within acceptable limits. As a transmission stopped subframe, for example, a MBSFN (MBMS (Multimedia Broadcast and Multicast Service) over a Single Frequency Network) subframe or an ABS (Almost Blank Subframe) may be used.
When setting an ABS, it has been studied to set the ABS at intervals of 8 msec, as illustrated in FIG. 3, in consideration of the period 8 msec of UL HARQ (Uplink Hybrid ARQ) in a macro cell/micro cell (pico cell) (the ABSs are indicated by open boxes in radio frames of the macro cell in FIG. 3). In this case, the micro base station (pico base station) transmits CSI-RSs (Channel State Information-Reference Signals) at intervals of 8 msec where the ABSs are set in the macro base station.
When the CSI-RSs are thus transmitted at intervals of 8 msec, paging subframes (subframes to communicate modification in system by paging) (at least one subframe out of subframes #0, #4, #5 and #9 in each radio frame) transmitted at intervals of 10 msec collide with subframes including CSI-RSs, and if they are transmitted as they are, there may arise collision between them. Note that in FIG. 3, there may arise collision in subframes #0 and #4.
Paging is provided to notify incoming calls, modification in system or the like and requires higher quality. Therefore, it has been proposed to prevent the pico base station from transmitting CSI-RSs in all the subframes (subframes #0, #4, #5 and #9) where there may be collision of the paging subframes and CSI-RS subframes.